Greenhouse Gases
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The most significant greenhouse emission is really water vapour, not something produced directly by humankind in significant amounts. However, even slight increases in atmospheric levels of  carbon dioxide (CO2) can cause a substantial increase in temperature. 

Why is this? There are two reasons: First, although the concentrations of these gases are not nearly as large as that of oxygen and nitrogen (the main constituents of the atmosphere), neither oxygen or nitrogen are greenhouse gases. This is because neither has more than two atoms per molecule and so they lack the internal vibrational modes that molecules with more than two atoms have. Both water and CO2, for example, have these "internal vibrational modes", and these vibrational modes can absorb and reradiate infrared radiation, which causes the greenhouse effect. 

Secondly,  CO2 tends to remain in the atmosphere for a very long time (time scales in the hundreds of years). Water vapor, on the other hand, can easily condense or evaporate, depending on local conditions. Water vapor levels therefore tend to adjust quickly to the prevailing conditions, such that the energy flows from the Sun and re-radiation from the Earth achieve a balance. CO2 tends to remain fairly constant and therefore behave as a controlling factor, rather than a reacting factor. additional greenhouse emission suggests that that the balance happens at higher temperatures and water vapor levels.  
On Earth, present amounts of greenhouse gases have a mean warming result of regarding thirty three °C (59 °F). while not the Earth's atmosphere, the Earth's average temperature would be below the phase change temperature of water.The major greenhouse gases area unit water vapour, that causes regarding 36–70% of the greenhouse effect; greenhouse emission (CO2), that causes 9–26%; alkane (CH4), that causes 4–9%; and gas (O3), that causes 3–7%. Clouds additionally have an effect on the radiation balance through cloud forcings the same as greenhouse gases.

Human activity since the economic Revolution has enlarged the number of greenhouse gases within the atmosphere, resulting in enlarged radiative forcing from greenhouse emission, methane, tropospheric gas, CFCs and inhalation anaesthetic. per work revealed in 2007, the concentrations of greenhouse emission and alkane have enlarged by 12 months and 148% severally since 1750.These levels area unit abundant more than at any time throughout the last 800,000 years, the amount that reliable knowledge has been extracted from ice cores. Less direct geologic proof indicates that greenhouse emission values more than this were last seen regarding twenty million years past.